Apparatus and methods for front-end systems with reactive loopback

ABSTRACT

Apparatus and methods for front-end systems with reactive loopback are provided. In certain configurations, a front-end system includes a transmit port that receives a transmit signal, an antenna port, a receive port, and an antenna switch connected along a transmit path between the transmit port and the antenna port. The front-end system further includes a loopback circuit including a reactive loopback impedance and a back switch electrically connected in series in a loopback path between the transmit port and the receive port. The loopback circuit provides a portion of the transmit signal to the receive port when the back switch is activated. Using reactive loopback impedance in the loopback circuit reduces an insertion loss of the transmit path relative to a configuration using resistive loopback impedance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 ofU.S. Provisional Patent Application No. 62/433,385, filed Dec. 13, 2016and titled “APPARATUS AND METHODS FOR FRONT-END SYSTEMS WITH REACTIVELOOPBACK,” which is herein incorporated by reference in its entirety.

BACKGROUND

Technical Field

Embodiments of the invention relate to electronic systems, and inparticular, to radio frequency (RF) electronics.

Description of the Related Technology

A radio frequency (RF) communication device can include a front-endsystem for processing signals communicated between a transceiver and anantenna of the RF system. A front-end system can provide a variety offunctionalities, such as amplifying signals for transmission, amplifyingreceived signals, filtering signals, switching or selection of signals,duplexing of signals, or multiplexing of signals (for instance,diplexing or triplexing)

Examples of RF electronics with one or more front-end systems include,but are not limited to, mobile phones, tablets, base stations, networkaccess points, customer-premises equipment (CPE), laptops, and wearableelectronics.

SUMMARY

In certain embodiments, the present disclosure relates to a front-endsystem. The front-end system includes an antenna port, a transmit portconfigured to receive a transmit signal, an antenna switch electricallyconnected along a transmit path between the transmit port and theantenna port, a receive port, and a loopback circuit including areactive loopback impedance and a back switch electrically connected inseries in a loopback path between the transmit port and the receiveport. The loopback circuit is configured to provide a portion of thetransmit signal to the receive port when the back switch is activated.

In some embodiments, the reactive loopback impedance includes acapacitor.

In a number of embodiments, the reactive loopback impedance includes twoor more capacitors electrically connected in series.

In accordance with various embodiments, the reactive loopback impedanceincludes two or more capacitor arrays electrically connected in series.

In several embodiments, the reactive loopback impedance includes aninductor.

In accordance with a number of embodiments, the loopback path does notinclude any explicit resistors.

According to some embodiments, the loopback circuit has an impedanceangle of at least 10 degrees at a frequency of interest.

In several embodiments, the frequency of interest is a carrier frequencyof the transmit signal.

In a number of embodiments, the front-end system further includes acontrol circuit configured to control the antenna switch and the backswitch.

In accordance with various embodiments, the front-end system furtherincludes a low noise amplifier electrically connected between theantenna port and the receive port in a receive path.

In some embodiments, the front-end system further includes an antenna DCblocking capacitor electrically connected between the antenna port andthe antenna switch, and a transmit port DC blocking capacitorelectrically connected between the antenna switch and the transmit port.

In a number of embodiments, the loopback circuit includes a receive portDC blocking capacitor, and the back switch is electrically connectedbetween the reactive loopback impedance and the receive port DC blockingcapacitor.

In several embodiments, the loopback circuit includes a terminationcircuit electrically connected between the back switch and ground.

In certain embodiments, the present disclosure relates to a method ofproviding transmit loopback in a front-end system. The method includesreceiving a transmit signal at a transmit port, providing the transmitsignal to an antenna port via a transmit path that includes an antennaswitch, controlling an amount of isolation between the transmit path anda receive port using a loopback circuit, the loopback circuit includinga reactive loopback impedance and a back switch electrically connectedin series between the transmit port and the receive port, and providinga portion of the transmit signal to the receive port when the backswitch is activated.

In some embodiments, controlling the amount of isolation between thetransmit path and the receive port includes using the loopback circuitto provide an impedance angle of at least 10 degrees at a carrierfrequency of the transmit signal.

In certain embodiments, the present disclosure relates to a wirelesscommunication device. The wireless communication device includes anantenna, a receive circuit configured to process a receive signal, atransmit circuit configured to generate a transmit signal, and afront-end system including an antenna port electrically connected to theantenna, a transmit port configured to receive the transmit signal fromthe transmit circuit, a receive port configured to provide the receivesignal to the receive circuit, and a loopback circuit including areactive loopback impedance and a back switch electrically connected inseries between the transmit port and the receive port. The loopbackcircuit is configured to provide a portion of the transmit signal to thereceive port when the back switch is activated.

In a number of embodiments, the reactive loopback impedance includes acapacitor.

In various embodiments, the reactive loopback impedance includes two ormore capacitors electrically connected in series.

In accordance with several embodiments, the reactive loopback impedanceincludes two or more capacitor arrays electrically connected in series.

In some embodiments, the reactive loopback impedance includes aninductor.

According to a number of embodiments, the loopback path does not includeany explicit resistors.

In several embodiments, the loopback circuit has an impedance angle ofat least 10 degrees at a frequency of interest.

In accordance with some embodiments, the frequency of interest is acarrier frequency of the transmit signal.

In various embodiments, the front-end system further includes a lownoise amplifier electrically connected between the antenna port and thereceive port in a receive path.

In some embodiments, the front-end system further includes an antenna DCblocking capacitor, an antenna switch, and a transmit port DC blockingcapacitor electrically connected in series between the antenna port andthe transmit port.

In a number of embodiments, the front-end system further includes acontrol circuit configured to control the antenna switch and the backswitch.

In several embodiments, the loopback circuit includes a receive port DCblocking capacitor, the back switch electrically connected between thereactive loopback impedance and the receive port DC blocking capacitor.

In a number of embodiments, the loopback circuit includes a terminationcircuit electrically connected between the back switch and ground.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of this disclosure will now be described, by way ofnon-limiting example, with reference to the accompanying drawings.

FIG. 1 is a schematic diagram of one embodiment of a front-end systemwith reactive loopback.

FIG. 2 is a schematic diagram of another embodiment of a front-endsystem with reactive loopback.

FIG. 3 is a schematic diagram of another embodiment of a front-endsystem with reactive loopback.

FIG. 4A is a schematic diagram of another embodiment of a front-endsystem with reactive loopback.

FIG. 4B is a schematic diagram of another embodiment of a front-endsystem with reactive loopback.

FIG. 4C is a schematic diagram of another embodiment of a front-endsystem with reactive loopback.

FIG. 4D is a schematic diagram of another embodiment of a front-endsystem with reactive loopback.

FIG. 4E is a schematic diagram of another embodiment of a front-endsystem with reactive loopback.

FIG. 4F is a schematic diagram of another embodiment of a front-endsystem with reactive loopback.

FIG. 5A is a schematic diagram of another embodiment of a front-endsystem with reactive loopback.

FIG. 5B is a schematic diagram of another embodiment of a front-endsystem with reactive loopback.

FIG. 5C is a schematic diagram of another embodiment of a front-endsystem with reactive loopback.

FIG. 6A is a graph of one example of insertion loss increase versusloopback isolation.

FIG. 6B is a graph of another example of insertion loss increase versusloopback isolation.

FIG. 7A is a graph of one example of insertion loss versus impedanceangle for a loopback isolation of about 25 decibels (dB).

FIG. 7B is a graph of one example of insertion loss versus impedanceangle for a loopback isolation of about 37.16 dB.

FIG. 8A is a schematic diagram of one embodiment of a front-end systemwith multiple loopbacks and a shared back switch.

FIG. 8B is a schematic diagram of another embodiment of a front-endsystem with multiple loopbacks and a shared back switch.

FIG. 8C is a schematic diagram of another embodiment of a front-endsystem with multiple loopbacks and a shared back switch.

FIG. 9 is a schematic diagram of another embodiment of a front-endsystem with multiple loopbacks and a shared back switch.

FIG. 10 is a schematic diagram of another embodiment of a front-endsystem with multiple loopbacks and a shared back switch.

FIG. 11 is a schematic diagram of another embodiment of a front-endsystem with multiple loopbacks and a shared back switch.

FIG. 12 is a schematic diagram of one embodiment of a front-end systemwith reactive loopback and with multiple loopbacks sharing a backswitch.

FIG. 13A is a schematic diagram of one embodiment of a mobile device.

FIG. 13B is a schematic diagram of one embodiment of a base station.

FIG. 14 is a schematic diagram of one embodiment of a radio frequencysystem.

FIG. 15A is a schematic diagram of one embodiment of a packaged module.

FIG. 15B is a schematic diagram of a cross-section of the packagedmodule of FIG. 15A taken along the lines 15B-15B.

FIG. 16 is a schematic diagram of another embodiment of a mobile device.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

The following detailed description of certain embodiments presentsvarious descriptions of specific embodiments. However, the innovationsdescribed herein can be embodied in a multitude of different ways, forexample, as defined and covered by the claims. In this description,reference is made to the drawings where like reference numerals canindicate identical or functionally similar elements. It will beunderstood that elements illustrated in the figures are not necessarilydrawn to scale. Moreover, it will be understood that certain embodimentscan include more elements than illustrated in a drawing and/or a subsetof the elements illustrated in a drawing. Further, some embodiments canincorporate any suitable combination of features from two or moredrawings.

Examples of Front-End Systems with Reactive Loopback

A front-end system can be used to control access of transmit and receivecircuitry to an antenna. For example, the front-end system can includetransmit and receive ports for handling radio frequency (RF) signals,including, but not limited to, wireless local area network (WLAN)signals, Bluetooth signals, and/or cellular signals. Additionally, thefront-end system can also include an antenna port and switches used toconnect a particular transmit and/or receive port to the antenna port.To enhance integration, a front-end system can include a low noiseamplifier (LNA), a power amplifier, and/or other circuitry integratedwith the switches. Front-end systems can be used in a wide variety ofapplications, including, but not limited to, mobile phones (forinstance, smartphones or handsets), tablets, base stations, networkaccess points, customer-premises equipment (CPE), laptops, and wearableelectronics.

In certain applications, it is desirable to provide loopback from aparticular transmit port to a receive port, thereby providing a portionof a transmit signal to the receive port. Implementing a front-endsystem with loopback can provide a number of benefits. For instance,loopback can be used when calibrating a transceiver for misalignments orimpairments, for performing digital pre-distortion (DPD), and/or for avariety of other purposes.

Apparatus and methods for front-end systems with reactive loopback areprovided. In certain configurations, a front-end system includes atransmit port that receives a transmit signal, an antenna port, areceive port, and an antenna switch connected along a transmit pathbetween the transmit port and the antenna port. The front-end systemfurther includes a loopback circuit including a reactive loopbackimpedance and a back switch electrically connected in series in aloopback path between the transmit port and the receive port. Theloopback circuit provides a portion of the transmit signal to thereceive port when the back switch is activated. Using reactive loopbackimpedance in the loopback circuit reduces an insertion loss of thetransmit path relative to a configuration using resistive loopbackimpedance.

A front-end system can be switchable between multiple modes, including,for example, one or more transmit modes and one or more receives modes.Additionally, loopback can be provided for one or more modes of thefront-end system.

A loopback circuit can be implemented to provide a particular amount offixed isolation, corresponding to a desired proportion of a transmitsignal to provide to the receive port. In certain applications, thedesired amount of fixed isolation is relatively small, which results ina relatively strong tradeoff between the amount of isolation and theresulting insertion loss to the transmit path due to the presence of theloopback circuit.

A resistor can be provided between a pair of ports to achieve a desiredloopback coupling factor between the ports. To decrease the amount ofisolation provided by the loopback, the resistance of the loopbackresistor can be decreased to allow a greater amount of signal to leakthrough.

However, resistance provides lossy impedance. For example, a currentflowing through a resistor leads to resistive heating and acorresponding energy loss. Accordingly, using resistive loopbackimpedance can lead to an increase in insertion loss beyond a predictedinsertion loss from three-port network theory.

In certain implementations herein, a loopback circuit includes aloopback impedance that is mostly reactive. In one embodiment, a totalloopback impedance of a loopback circuit has an impedance angle of atleast 10 degrees, or more particularly at least 20 degrees at afrequency of interest (for instance, a carrier frequency of the transmitsignal). In one embodiment, the loopback path does not include anyexplicit resistors.

By using reactive loopback impedance that is ideally lossless, insertionloss of a front-end with loopback functionality can be significantlyimproved relative to an implementation using resistive loopback. Thebenefits of reactive loopback can be particularly beneficial forimplementations in which the desired amount of isolation provided by theloopback circuit is relatively low, for instance, 25 dB or smaller.

The loopback circuits herein can be used for a variety of radiofrequency signals, including, but not limited to, signals in the rangeof 30 kHz to 300 gigahertz (GHz), such as in the range of about 450 MHzto about 6 GHz for certain communications standards. In one specificexample, a loopback circuit with reactive loopback impedance is used ina wireless local area network (WLAN) application, for instance, 2.4 GHzand 5 GHz Wi-Fi frequency bands.

FIG. 1 is a schematic diagram of one embodiment of a front-end system 10with reactive loopback. The front-end system 10 includes an antenna portANT, a transmit port TX, a receive port RX, a transmit port DC blockingcapacitor 1, an antenna port DC blocking capacitor 2, an antenna switch3, a loopback circuit 5, and a control circuit 6. The loopback circuit 5includes a reactive loopback impedance 11, a back switch 12, a receiveport DC blocking capacitor 13, and a termination circuit 14.

Although one embodiment of a front-end system including reactiveloopback is shown in FIG. 1, the teachings herein are applicable tofront-end systems implemented in a wide variety of ways.

For example, a front-end system with reactive loopback can includeadditional ports, paths, switches, and/or other circuitry, including,for instance, LNAs and/or power amplifiers. Moreover, the teachingsherein are applicable to configurations using a different arrangement ofcomponents, such as different implementations of switching, DC blocking,and/or termination. Accordingly, other configurations are possible.

As shown in FIG. 1, the transmit port DC blocking capacitor 1, theantenna switch 3, and the antenna port DC blocking capacitor 2 areelectrically connected in series along a transmit path between thetransmit port TX and the antenna port ANT. Additionally, the loopbackcircuit 5 is connected between a node of the transmit path and thereceive port RX. As shown in FIG. 1, the loopback circuit 5 is in shuntwith respect to the transmit path. When the back switch 12 is activated(thereby selecting or activating loopback), the loopback circuit 5provides a portion of the transmit signal received on the transmit portTX to the receive port RX.

In the illustrated embodiment, the reactive loopback impedance 11, theback switch 12, and the receive port DC blocking capacitor 13 areelectrically connected in series along a loopback path. Additionally,the termination circuit 14 is connected in shunt to the loopback path.As shown in FIG. 1, the termination circuit 14 is connected betweenground and a node that is between the reactive loopback impedance 11 andthe back switch 12.

The termination circuit 14 aids in terminating the receive port RX whenthe back switch 12 is activated. Thus, the termination circuit 14 canenhance matching of the receive port RX. Although one example placementof a termination circuit is shown, termination can be provided in otherways. In another embodiment, a termination circuit is omitted.

The control circuit 6 controls the mode of the front-end system 10 byopening or closing the antenna switch 3 and/or the back switch 12. Forexample, by controlling the state of the antenna switch 3, the transmitpath between the antenna terminal ANT and the transmit terminal TX canbe selectively activated. Additionally, by controlling the state of theback switch 12, loopback from the transmit path to the receive port RXcan be selectively activated.

Although one example of switching is shown, other implementations ofswitches are possible, including, but not limited to, other switchplacements and/or other numbers of switches. For clarity of the figures,control signals from the control circuit 6 for opening or closing theswitches are not illustrated.

Although not shown in FIG. 1 for clarity of the figures, the front-endsystem 10 can include additional signal paths that are selectable viaswitches and/or other circuitry.

The illustrated loopback circuit 5 includes the reactive loopbackimpedance 11. By using the reactive loopback impedance 11, an insertionloss between the transmit port TX and the antenna port ANT can bereduced relative to an implementation using resistive loopback. Forexample, a current flowing through a resistor leads to resistive heatingand corresponding energy loss. Accordingly, the illustrated embodimentexhibits superior insertion loss performance.

Additional details of the front-end system 10 can be as describedearlier.

FIG. 2 is a schematic diagram of another embodiment of a front-endsystem 20 with reactive loopback. The front-end system 20 includes anantenna port ANT, a transmit port TX, a receive port RX, a transmit portDC blocking capacitor 1, an antenna port DC blocking capacitor 2, anantenna switch 3, a loopback circuit 5, a control circuit 6, an LNAinput DC blocking capacitor 21, an LNA output DC blocking capacitor 22,an LNA input switch 23, an LNA output switch 24, and an LNA 25.

The front-end system 20 of FIG. 2 is similar to the front-end system 10of FIG. 1, except that the front-end system 20 further includes areceive path from the antenna port ANT to the receive port RX via theLNA 25. The front-end systems herein can include a wide variety ofports, switches, paths, and circuitry. Although one specificimplementation of an LNA receive path is shown in FIG. 2, otherimplementations are possible. Any of the front-end systems herein can beadapted to include a receive path with or without an LNA. Additionally,any of the front-end systems herein can be adapted to include a poweramplifier in the transmit path.

Additional details of the front-end system 20 can be as describedearlier.

FIG. 3 is a schematic diagram of another embodiment of a front-endsystem 30 with reactive loopback. The front-end system 30 includes anantenna port ANT, a transmit port TX, a receive port RX, a transmit portDC blocking capacitor 1, an antenna port DC blocking capacitor 2, anantenna switch 33, a loopback circuit 35, and a control circuit 36. Theloopback circuit 35 includes a reactive loopback impedance 11, a backswitch 42, a receive port DC blocking capacitor 13, and a terminationcircuit 44.

The front-end system 30 of FIG. 3 is similar to the front-end system 10of FIG. 1, except that the front-end system 30 of FIG. 3 illustratesspecific termination circuit and switch implementations.

In particular, the termination circuit 44 includes a terminationresistor 71 and a termination capacitor 72 electrically connected inseries between a node of the loopback path and ground. In certainembodiments, the termination resistor 71 provides a desired terminationimpedance (for instance, about 50 Ohm), and the termination capacitor 72is sufficiently large (for instance, at least 10 pF) to provide lowimpedance at the frequency of interest such that the terminationresistor 71 provides the proper amount of termination.

The illustrated antenna switch 33 includes a field-effect transistor(FET) 51, a channel biasing resistor 52, a biasing inverter 53, aninverter output resistor 54, and a gate resistor 55. In the illustratedembodiment, a transmit selection control signal C_(TX) from the controlcircuit 36 controls the FET 51 to selectively activate the transmit pathby coupling or decoupling the transmit port TX and the antenna port ANTfrom one another. Additionally, the biasing inverter 53 controls thechannel voltage of the FET 51 based on a state of the transmit selectioncontrol signal C_(TX). The biasing inverter 53 provides negativegate-to-source voltage and high channel impedance when the FET 51 isturned off.

Although one specific implementation of an antenna switch is shown, theteachings herein are applicable to switches implemented in a widevariety of ways. For example, other types of transistors can be used.Moreover, a switch can be implemented without transistors, for instance,a pin diode switch and/or a microelectromechanical switch.

The illustrated back switch 42 includes a first FET 61, a second FET 62,a first channel biasing resistor 63, a second channel biasing resistor64, a biasing inverter 65, an inverter output resistor 66, a first gateresistor 67, and a second gate resistor 68. As shown in FIG. 3, aloopback selection control signal C_(LB) from the control circuit 36controls the first FET 61 and the second FET 62 to selectively activatethe loopback path. The back switch 42 is similar to the antenna switch33, except that the back switch 42 includes multiple FETs in series toincrease switch linearity and power handling capabilities. For example,two or more FETs can be included in series to achieve target switchperformance specifications.

Although one specific implementation of a back switch is shown, theteachings herein are applicable to switches implemented in a widevariety of ways, including, but not limited to, other types oftransistor switches.

Additional details of the front-end system 30 can be as describedearlier.

FIGS. 4A-4F show specific implementations of a reactive loopbackimpedance. Although various examples are illustrated, reactive loopbackimpedance can be implemented in a wide variety of ways. For example, areactive loopback impedance can be implemented with any impedanceincluding an imaginary or reactive component. In certainimplementations, a combination of components is included in the reactiveloopback impedance, which can aid in enhancing robustness tomanufacturing and/or a number of other advantages.

For example, in one embodiment, a reactive loopback impedance providesnot only a desired amount of loopback isolation at the frequency ofinterest, but also serves as a filter. For example, using reactiveloopback impedance also as a filter can aid in providing wide bandwidthoperation relative to a single component that provides a first-orderslope to the loopback circuit's frequency response.

FIG. 4A is a schematic diagram of another embodiment of a front-endsystem 100 with reactive loopback. The front-end system 100 includes anantenna port ANT, a transmit port TX, a receive port RX, a transmit portDC blocking capacitor 1, an antenna port DC blocking capacitor 2, anantenna switch 3, a loopback circuit 105, and a control circuit 6. Theloopback circuit 105 includes a reactive loopback impedance 111, a backswitch 12, a receive port DC blocking capacitor 13, and a terminationcircuit 14. The reactive loopback impedance 111 includes a capacitor112.

The front-end system 100 of FIG. 4A is similar to the front-end system10 of FIG. 1, except that the front-end system 100 illustrates aspecific implementation of a reactive loopback impedance.

In certain implementations, a capacitor is used to provide substantiallyall of a loopback isolation value. For example, a capacitor can providean impedance of 1/(j*ω*C_(LBK)), where j is the imaginary unit, ω isangular frequency, and C_(LBK) is the loopback capacitance. Sinceimpedance increases inversely with capacitance, a relatively smallcapacitor can be used to achieve a relatively large loopback impedance.Accordingly, relatively low amounts of loopback isolation can beachieved using relatively small capacitors.

In one embodiment, a total capacitance of a reactive loopback circuit isin the range of about 25 femtofarad (fF) and 300 fF. However, othercapacitance values are possible, such as capacitance values that dependon a desired amount of isolation and/or operating frequency. In certainapplications, a desired amount of capacitance can be less than asmallest single capacitor (for instance, a metal-insulator-metal or MIMcapacitor) can achieve in a given processing technology. In certainimplementations, multiple components, such as multiple capacitors inseries, can be used to achieve a desired impedance value.

Additional details of the front-end system 100 can be as describedearlier.

FIG. 4B is a schematic diagram of another embodiment of a front-endsystem 120 with reactive loopback. The front-end system 120 includes anantenna port ANT, a transmit port TX, a receive port RX, a transmit portDC blocking capacitor 1, an antenna port DC blocking capacitor 2, anantenna switch 3, a loopback circuit 125, and a control circuit 6. Theloopback circuit 125 includes a loopback impedance 121, a back switch12, a receive port DC blocking capacitor 13, and a termination circuit14. The loopback impedance 121 includes a resistor 113 and a reactiveloopback capacitor 112.

The front-end system 120 of FIG. 4B is similar to the front-end system100 of FIG. 4A, except that the front-end system 120 of FIG. 4B includesa resistor 113 in series with the capacitor 112.

In certain implementations herein, a loopback circuit includes aloopback impedance that is mostly reactive. However, a resistor can beused in part to achieve a desired loopback impedance. For example,including a resistor to provide some loopback impedance can improvefrequency characteristics, for instance, by providing lower slope overfrequency for loopback and/or insertion loss. In one embodiment, a totalloopback impedance of a loopback circuit has an impedance angle of atleast 10 degrees, or more particularly at least 20 degrees at afrequency of interest.

Additional details of the front-end system 120 can be as describedearlier.

FIG. 4C is a schematic diagram of another embodiment of a front-endsystem 140 with reactive loopback. The front-end system 140 includes anantenna port ANT, a transmit port TX, a receive port RX, a transmit portDC blocking capacitor 1, an antenna port DC blocking capacitor 2, anantenna switch 3, a loopback circuit 145, and a control circuit 6. Theloopback circuit 145 includes a reactive loopback impedance 141, a backswitch 12, a receive port DC blocking capacitor 13, and a terminationcircuit 14. The reactive loopback impedance 141 includes capacitors 151,152, and 153 in series.

The front-end system 140 of FIG. 4C is similar to the front-end system10 of FIG. 1, except that the front-end system 140 illustrates aspecific implementation of a reactive loopback impedance.

In particular, the illustrated reactive loopback impedance 141 includescapacitors 151, 152, and 153 in series, which aids in achieving highloopback impedance and corresponding low isolation using relativelysmall capacitors. Although an example with three capacitors in series isshown, more or fewer capacitors can be included in series to achieve anamount of desired reactive loopback impedance. For example, in oneembodiment, two capacitors are included in series. In anotherembodiment, four or more capacitors are included in series to achievedesired reactive loopback impedance.

Additional details of the front-end system 140 can be as describedearlier.

FIG. 4D is a schematic diagram of another embodiment of a front-endsystem 160 with reactive loopback. The front-end system 160 includes anantenna port ANT, a transmit port TX, a receive port RX, a transmit portDC blocking capacitor 1, an antenna port DC blocking capacitor 2, anantenna switch 3, a loopback circuit 165, and a control circuit 6. Theloopback circuit 165 includes a reactive loopback impedance 161, a backswitch 12, a receive port DC blocking capacitor 13, and a terminationcircuit 14.

The front-end system 160 of FIG. 4D is similar to the front-end system10 of FIG. 1, except that the front-end system 160 illustrates aspecific implementation of a reactive loopback impedance.

In particular, the illustrated reactive loopback impedance 161 includesa first capacitor array 171-173, a second capacitor array 181-183, and athird capacitor array 191-193 in series. In this example, each capacitorarray includes multiple capacitors in parallel, which aids in reducingmanufacturing variation, thereby achieving tighter tolerance withrespect the amount of isolation provided by loopback. Moreover,connecting capacitor arrays in series achieves high loopback impedanceand corresponding low isolation using relatively small capacitors.

Accordingly, the illustrated embodiment provides robustness tomanufacturing variation while achieving compact layout. Although anexample with three capacitors arrays in series and three capacitors perarray are shown, any suitable number of capacitor arrays can be used,and different number of capacitors can be included in each array. Thus,more or fewer capacitors can be included in parallel with one another ina given array, and/or more or fewer capacitor arrays can be included inseries to achieve a desired amount of reactive loopback. Moreover, thenumber of capacitors and/or the capacitance values used in each arrayneed not be equal.

Additional details of the front-end system 160 can be as describedearlier.

FIG. 4E is a schematic diagram of another embodiment of a front-endsystem 200 with reactive loopback. The front-end system 200 includes anantenna port ANT, a transmit port TX, a receive port RX, a transmit portDC blocking capacitor 1, an antenna port DC blocking capacitor 2, anantenna switch 3, a loopback circuit 205, and a control circuit 6. Theloopback circuit 205 includes a reactive loopback impedance 201, a backswitch 12, a receive port DC blocking capacitor 13, and a terminationcircuit 14. The reactive loopback impedance 201 includes an inductor202.

The front-end system 200 of FIG. 4E is similar to the front-end system10 of FIG. 1, except that the front-end system 200 illustrates aspecific implementation of a reactive loopback impedance.

In certain implementations, an inductor is used to provide substantiallyall of a loopback isolation value. However, other implementations arepossible, such as implementations in which a capacitor is used toprovide substantially all of a loopback isolation value or a combinationof an inductor and a capacitor are used to provide substantially all ofa loopback isolation value. Moreover, the teachings herein areapplicable to a reactive loopback impedance implemented using multipleinductors and/or capacitors.

The inductor 202 can have any suitable inductance value, such aninductance value selected based on a desired amount of isolation and/oroperating frequency.

Additional details of the front-end system 200 can be as describedearlier.

FIG. 4F is a schematic diagram of another embodiment of a front-endsystem 220 with reactive loopback. The front-end system 220 includes anantenna port ANT, a transmit port TX, a receive port RX, a transmit portDC blocking capacitor 1, an antenna port DC blocking capacitor 2, anantenna switch 3, a loopback circuit 225, and a control circuit 6. Theloopback circuit 225 includes a loopback impedance 221, a back switch12, a receive port DC blocking capacitor 13, and a termination circuit14. The loopback impedance 221 includes a resistor 203 and a reactiveloopback inductor 202.

The front-end system 220 of FIG. 4F is similar to the front-end system200 of FIG. 4E, except that the front-end system 220 of FIG. 4F furtherincludes the resistor 203 in series with the inductor 202.

In certain implementations herein, a loopback circuit includes aloopback impedance that is mostly reactive. However, a resistor can beused in part to achieve a desired loopback impedance.

Additional details of the front-end system 220 can be as describedearlier.

FIG. 5A is a schematic diagram of another embodiment of a front-endsystem 230 with reactive loopback. The front-end system 230 includes anantenna port ANT, a transmit port TX, a receive port RX, a transmit portDC blocking capacitor 1, an antenna port DC blocking capacitor 2, anantenna switch 3, a control circuit 6, and a loopback circuit 235. Theloopback circuit 235 includes a reactive loopback impedance 11, a backswitch 12, a receive port DC blocking capacitor 13, a terminationcircuit 14, and an isolation switch 236. The reactive loopback impedance11 can be implemented in a wide variety of ways, including, but notlimited to, using any of implementations described above with respect toFIGS. 4A-4F.

The front-end system 230 of FIG. 5A is similar to the front-end system10 of FIG. 1, except that the front-end system 230 further includes theisolation switch 236. In the illustrated embodiment, the isolationswitch 236 is positioned between the termination circuit 14 and ground.

Including the isolation switch 236 can aid in further isolating thetransmit path from the loopback circuit 235 when loopback is deactivatedor disabled. For example, in certain implementations both the backswitch 12 and the isolation switch 236 are opened when loopback isdisabled or deactivated, and both closed when loopback is enabled oractivated. Including the isolation switch 236 can further reduceundesired loading of the transmit path by the loopback circuit 235 whenthe loopback circuit 235 is turned off.

FIG. 5B is a schematic diagram of another embodiment of a front-endsystem 240 with reactive loopback. The front-end system 240 includes anantenna port ANT, a transmit port TX, a receive port RX, a transmit portDC blocking capacitor 1, an antenna port DC blocking capacitor 2, anantenna switch 3, a control circuit 6, and a loopback circuit 245. Theloopback circuit 245 includes a reactive loopback impedance 11, a backswitch 12, a receive port DC blocking capacitor 13, a terminationcircuit 14, and an isolation switch 246. The reactive loopback impedance11 can be implemented in a wide variety of ways, including, but notlimited to, using any of implementations described above with respect toFIGS. 4A-4F.

The front-end system 240 of FIG. 5B is similar to the front-end system10 of FIG. 1, except that the front-end system 240 further includes theisolation switch 246. In the illustrated embodiment, the isolationswitch 246 is positioned between the reactive loopback impedance 11 andthe termination circuit 14. Including the isolation switch 246 can aidin further isolating the transmit path of the front-end system from theloopback circuit 245 when loopback is deactivated.

FIG. 5C is a schematic diagram of another embodiment of a front-endsystem 250 with reactive loopback. The front-end system 250 includes anantenna port ANT, a transmit port TX, a receive port RX, a transmit portDC blocking capacitor 1, an antenna port DC blocking capacitor 2, anantenna switch 3, a control circuit 6, and a loopback circuit 255. Theloopback circuit 255 includes a reactive loopback impedance 11, a backswitch 12, a receive port DC blocking capacitor 13, a terminationcircuit 14, and an isolation switch 256. The reactive loopback impedance11 can be implemented in a wide variety of ways, including, but notlimited to, using any of implementations described above with respect toFIGS. 4A-4F.

The front-end system 250 of FIG. 5C is similar to the front-end system10 of FIG. 1, except that the front-end system 250 further includes theisolation switch 256. In the illustrated embodiment, the isolationswitch 256 is positioned between the transmit path and the reactiveloopback impedance 11. Including the isolation switch 256 can aid infurther isolating the transmit path of the front-end system from theloopback circuit 255 when loopback is deactivated.

Although various examples of switch implementations are shown in FIGS.5A-5C, the teachings herein are applicable to front-end systemsincluding switches implemented in a wide variety of ways. For example,the number and placement of switches can be selected based on a widevariety of factors and considerations.

FIG. 6A is a graph 300 of one example of insertion loss increase versusloopback isolation. The graph 300 illustrates insertion loss increase indecibels (dB) (relative to a case of no loopback) versus the amount ofisolation of the loopback path in dB. The graph 300 includes a firstplot for a resistive loopback circuit simulated with parasiticcomponents extracted from layout and accounting for energy loss fromresistive heating. The graph 300 includes a second plot for theresistive loopback circuit based on a predicted insertion loss fromthree-port network theory.

As shown in FIG. 6A, using a resistive loopback impedance can lead to anincrease in insertion loss beyond the predicted insertion loss fromthree-port network theory.

FIG. 6B is a graph 330 of another example of insertion loss increaseversus loopback isolation. The graph 330 illustrates insertion lossincrease in dB (relative to a case of no loopback) versus the amount ofisolation of the loopback path in dB. The graph 330 includes first totenth plots 331-340, respectively, corresponding to a different amountof fixed capacitance, with the amount of resistance swept from 0Ω to 1kΩ to achieve different amounts of isolation. The first to tenth plots331-340 correspond to capacitance values of 133 fF, 145 fF, 160 fF, 185fF, 200 fF, 215 fF, 230 fF, 266 fF, 532 fF, and infinite (INF),respectively.

As shown in FIG. 6B, loss increases as the proportion of resistance usedto implement the loopback impedance is increased.

FIG. 7A is a graph 345 of one example of insertion loss versus impedanceangle for a loopback isolation of about 25 dB. Each point of the graph345 illustrates a different combination of resistance and capacitancevalues used to achieve 25 dB of loopback isolation at 2.4 GHz, within atolerance of +/−0.5 dB.

FIG. 7B is a graph 350 of one example of insertion loss versus impedanceangle for a loopback isolation of about 37.16 dB. Each point of thegraph 350 illustrate a different combination of resistance andcapacitance values used to achieve 37.16 dB of loopback isolation at 2.4GHz, within a tolerance of +/−0.35 dB.

As shown in FIGS. 7A and 7B, as impedance angle increases (indicating agreater proportion of reactive impedance relative to resistanceimpedance), insertion loss is improved. For instance, in the particularexample simulation of FIG. 7A, replacing a resistive impedance with areactive impedance achieves about 37 dB of isolation with about 0.6 dBof insertion loss. Additionally, in the particular example simulation ofFIG. 7B, replacing a resistive impedance with a reactive impedanceachieves about 25 dB of isolation with about 0.7 dB of insertion loss.In contrast, 25 dB loopback with a resistor suffers from an insertionloss that is about 0.5 dB poorer.

Although specific examples of simulation results for a front-end areshown in FIGS. 6A-7B, other simulations results are possible. Forexample, simulation results can vary based on a wide variety of factors,including, but not limited to, simulation parameters (includingoperating frequency), circuit topology, and/or fabrication processes.

Examples of Front-End Systems with a Shared Back Switch

In certain applications, it is desirable for a front-end system toinclude multiple loopbacks.

For example, a front-end system can include multiple transmit ports, andin can be desirable for loopback to be provided for two more of thetransmit ports. In such applications, the amount of desired isolationcan vary for each loopback. In one specific example, a front-end systemincludes a cellular transmit port with a desired 37 dB isolation to areceive port, a high power Bluetooth port with a desired 32 dB isolationto the receive port, and a low power Bluetooth port with a desired 25 dBisolation to the receive port.

Including multiple loopbacks in a front-end system can increase the areaand/or complexity of the front-end system. For example, circuitry foreach loopback, such as capacitors, can occupy a substantial amount ofchip area. Moreover, by providing multiple loopbacks to a common receiveport, layout resources near the receive port can be constrained.

Apparatus and methods for front-end systems with multiple loopbacks anda shared back switch are provided. In certain configurations, afront-end system includes a first transmit path between a first transmitport and an antenna port, a second transmit path between a secondtransmit port and the antenna port, a first loopback path from the firsttransmit path to a receive port, and a second loopback path from thesecond transmit path to the receive port. The first and second loopbackpaths operate with a shared back switch to reduce area and/or complexityof the front-end system. Additionally, each loopback path includes aloopback selection switch that operates in combination with the backswitch to select that particular loopback path. In certainimplementations, the front-end system includes one or more additionaltransmit paths and corresponding loopback paths that are connected tothe receive port via the shared back switch.

Using a shared back switch can provide a number of advantages. Forexample, using a shared back switch can reduce a number of terminationcircuits associated with the loopbacks. Since a termination circuit caninclude a relatively large termination capacitor for grounding atermination resistor to provide appropriate termination (for instance,50Ω), reducing a number of termination circuits can reduce chip area.Moreover, it is desirable for such termination capacitors to bepositioned relatively close in distance to the receive port, forinstance, within about 200 μm, thereby avoiding performance degradationarising from long signal routes and corresponding parasitics. Thus,eliminating termination capacitors can alleviate layout constraints inproximity to the receive port.

Accordingly, by utilizing a common back switch and a shared terminationcircuit across multiple loopback arms or paths, die area is reduced andlayout constraints are relaxed.

FIG. 8A is a schematic diagram of one embodiment of a front-end system500 with multiple loopbacks and a shared back switch. The front-endsystem 500 includes an antenna port ANT, a first transmit port TX1, asecond transmit port TX2, a receive port RX, a first transmit port DCblocking capacitor la, a second transmit port DC blocking capacitor 1 b,a first antenna port DC blocking capacitor 2 a, a second antenna port DCblocking capacitor 2 b, a first antenna switch 3 a, a second antennaswitch 3 b, a control circuit 6, a first loopback resistor 9 a, a secondloopback resistor 9 b, a receive port DC blocking capacitor 13, atermination circuit 14, a first loopback DC blocking capacitor 15 a, asecond loopback DC blocking capacitor 15 b, a first loopback selectionswitch 17 a, a second loopback selection switch 17 b, and a shared backswitch 18.

Although one embodiment of a front-end system with multiple loopbacksand a shared back switch is shown in FIG. 8A, the teachings herein areapplicable to front-end systems implemented in a wide variety of ways.For example, a front-end system with multiple loopbacks and a sharedback switch can include additional ports, paths, switches, and/or othercircuitry, including, for instance, LNAs and/or power amplifiers.Moreover, the teachings herein are applicable to configurations using adifferent arrangement of components, such as different implementationsof switching, DC blocking, and/or termination. Accordingly, otherconfigurations are possible.

As shown in FIG. 8A, the first transmit port DC blocking capacitor 1 a,the first antenna switch 3 a, and the first antenna port DC blockingcapacitor 2 a are electrically connected in series along a firsttransmit path between the first transmit port TX1 and the antenna portANT. Additionally, the second transmit port DC blocking capacitor 1 b,the second antenna switch 3 b, and the second antenna port DC blockingcapacitor 2 b are electrically connected in series along a secondtransmit path between the second transmit port TX2 and the antenna portANT.

The first loopback resistor 9 a, the first loopback selection switch 17a, the first loopback DC blocking capacitor 15 a, the shared back switch18, and the receive port DC blocking capacitor 13 are electricallyconnected in series along a first loopback path from the first transmitport TX1 to the receive port RX. Additionally, the second loopbackresistor 9 b, the second loopback selection switch 17 b, the secondloopback DC blocking capacitor 15 b, the shared back switch 18, and thereceive port DC blocking capacitor 13 are electrically connected inseries along a second loopback path from the second transmit port TX2 tothe receive port RX.

The first transmit port TX1 and the second transmit port TX2 can be usedfor transmission of signals of a wide variety of frequencies and/orsignal types. For example, each of the transmit ports can operate atdifferent frequencies and/or can receive transmit signals of differentprotocols. The impedances of the first and second loopback paths, suchas the resistances of the first loopback resistor 9 a and the secondloopback resistor 9 b, can be implemented to achieve a desiredisolation, which can be different for each loopback path. Although animplementation with resistive loopback impedance is shown, reactiveloopback impedance can also be used.

Moreover, the teachings herein are applicable to implementations inwhich one or more of the loopback resistors 9 a and 9 b are omitted infavor of including directional couplers. Accordingly, in certainembodiments, multiple directional couplers can operate using a sharedback switch.

As shown in FIG. 8A, the termination circuit 14 is shared amongst theloopback paths, and operates in shunt thereto. In the illustratedembodiment, the termination circuit 14 is connected between ground andan end or terminal of the shared back switch 18.

The termination circuit 14 aids in terminating the receive port RX whenthe shared back switch 18 is activated. Thus, the termination circuit 14can enhance matching of the receive port RX. Although one exampleplacement of a termination circuit is shown, termination circuits can beimplemented in other ways. In another embodiment, a termination circuitis omitted.

The control circuit 6 controls the mode of the front-end system 500,such as by opening or closing the illustrated switches. For example, thefirst and second antenna switches 3 a and 3 b can be turned on to selectthe first and second transmit paths, respectively. Additionally, thefirst loopback selection switch 17 a and the shared back switch 18 canbe turned on to select the first loopback path, and the second loopbackselection switch 17 b and the shared back switch 18 can be turned on toselect the second loopback path. By controlling the state of theswitches, the control circuit 6 can activate the desired path(s) of thefront-end system 500.

Using the shared back switch 18 and the shared termination circuit 14across multiple loopback paths reduces die area and layout constraintsnear the receive port RX.

For example, a termination circuit for a loopback path can include atermination resistor and a relatively large termination capacitor forgrounding the termination resistor at the frequency of interest. Byusing the shared termination circuit 14, a number of terminationcapacitors can be reduced. Moreover, since it is desirable for suchtermination capacitors to be positioned relatively close in distance tothe receive port to avoid degradation in termination arising from longconductive routes, eliminating termination capacitors can alleviatelayout constraints in proximity to the receive port RX.

Additional details of the front-end system 500 can be as describedearlier.

FIG. 8B is a schematic diagram of another embodiment of a front-endsystem 520 with multiple loopbacks and a shared back switch. Thefront-end system 520 includes an antenna port ANT, a first transmit portTX1, a second transmit port TX2, a receive port RX, a first transmitport DC blocking capacitor 1 a, a second transmit port DC blockingcapacitor 1 b, a first antenna port DC blocking capacitor 2 a, a secondantenna port DC blocking capacitor 2 b, a first antenna switch 3 a, asecond antenna switch 3 b, a control circuit 6, a first loopbackresistor 9 a, a second loopback resistor 9 b, a receive port DC blockingcapacitor 13, a termination circuit 44, a first loopback DC blockingcapacitor 15 a, a second loopback DC blocking capacitor 15 b, a firstloopback selection switch 17 a, a second loopback selection switch 17 b,and a shared back switch 18.

The front-end system 520 of FIG. 8B is similar to front-end system 500of FIG. 8A, except that the front-end system 520 of FIG. 8B illustratesa specific termination circuit implementation.

In particular, the termination circuit 44 includes a terminationresistor 71 and a termination capacitor 72 electrically connected inseries between a common node to the loopback paths and ground. Incertain embodiments, the termination resistor 71 provides a desiredtermination impedance (for instance, about 50 Ohm), and the terminationcapacitor 72 can be sufficiently large (for instance, at least 10 pF) toprovide low impedance at the frequency of interest such that thetermination resistor 71 provides the proper amount of terminationimpedance.

When the shared back switch 18 is turned on, the termination circuit 44operates to provide termination to the receive port RX. In one example,the front-end system 520 operates in a 50Ω signaling environment, andthe termination resistor 71 is selected to be about 50Ω a To provideproper termination at the frequencies of interest, the terminationcapacitor 72 and the receive port DC blocking capacitor 13 can berelatively large. For instance, in one specific example, the terminationcapacitor 72 is selected to be about 12 pF and the receive port DCblocking capacitor 13 is selected to be about 6 pF.

By sharing the back switch 18 and the termination circuit 44 amongstmultiple loopbacks, die area and/or layout constraints are reduced.

Additional details of the front-end system 520 can be as describedearlier.

FIG. 8C is a schematic diagram of another embodiment of a front-endsystem 530 with multiple loopbacks and a shared back switch. Thefront-end system 530 includes an antenna port ANT, a first transmit portTX1, a second transmit port TX2, a receive port RX, a first transmitport DC blocking capacitor 1 a, a second transmit port DC blockingcapacitor 1 b, a first antenna port DC blocking capacitor 2 a, a secondantenna port DC blocking capacitor 2 b, a first antenna switch 3 a, asecond antenna switch 3 b, a control circuit 6, a first directionalcoupler 19 a, a second directional coupler 19 b, a receive port DCblocking capacitor 13, a termination circuit 14, a first loopback DCblocking capacitor 15 a, a second loopback DC blocking capacitor 15 b, afirst loopback selection switch 17 a, a second loopback selection switch17 b, and a shared back switch 18.

The front-end system 530 of FIG. 8C is similar to the front-end system500 of FIG. 8A, except that the front-end system 530 of FIG. 8C omitsthe first loopback resistor 9 a and the second loopback resistor 9 b infavor of including the first directional coupler 19 a and the seconddirectional coupler 19 b, respectively.

FIG. 9 is a schematic diagram of another embodiment of a front-endsystem 540 with multiple loopbacks and a shared back switch. Thefront-end system 540 includes an antenna port ANT, a first transmit portTX1, a second transmit port TX2, a third transmit port TX3, a receiveport RX, a first transmit port DC blocking capacitor 1 a, a secondtransmit port DC blocking capacitor 1 b, a third transmit port DCblocking capacitor 1 c, a first antenna port DC blocking capacitor 2 a,a second antenna port DC blocking capacitor 2 b, a third antenna port DCblocking capacitor 2 c, a first antenna switch 3 a, a second antennaswitch 3 b, a third antenna switch 3 c, a control circuit 6, a firstloopback resistor 9 a, a second loopback resistor 9 b, a third loopbackresistor 9 c, a receive port DC blocking capacitor 13, a terminationcircuit 14, a first loopback DC blocking capacitor 15 a, a secondloopback DC blocking capacitor 15 b, a third loopback blocking capacitor15 c, a first loopback selection switch 17 a, a second loopbackselection switch 17 b, a third loopback selection switch 17 c, and ashared back switch 18.

The front-end system 540 of FIG. 9 is similar to front-end system 500 ofFIG. 8A, except that the front-end system 540 of FIG. 9 includes anadditional or third transmit path and a corresponding loopback path thatis connected to the receive port RX via the shared back switch 18.

As shown in FIG. 9, the third transmit port DC blocking capacitor 1 c,the third antenna switch 3 c, and the third antenna port DC blockingcapacitor 2 c are electrically connected in series along a thirdtransmit path between the third transmit port TX3 and the antenna portANT. Additionally, the third loopback resistor 9 c, the third loopbackselection switch 17 c, the third loopback DC blocking capacitor 15 c,the shared back switch 18, and the receive port DC blocking capacitor 13are electrically connected in series along a third loopback path fromthe third transmit port TX3 to the receive port RX.

Accordingly, the illustrated front-end system 540 includes threeloopback paths that share the back switch 18. Although an example withthree loopback paths is shown, a back switch can be shared amongst moreor fewer loopback paths. In another example, four or more loopback pathsshare a back switch.

Additionally, the control circuit 6 can control the shared back switch18 and the loopback selection switches 17 a-17 c to select a particularloopback path. For example, to select the first loopback path, thecontrol circuit 6 can turn on the shared back switch 18 and the firstloopback selection switch 17 a and turn off the second and thirdloopback selection switches 17 b, 17 c. Additionally, to select thesecond loopback path, the control circuit 6 can turn on the shared backswitch 18 and the second loopback selection switch 17 b and turn off thefirst and third loopback selection switches 17 a, 17 c. Furthermore, toselect the third loopback path, the control circuit 6 can turn on theshared back switch 18 and the third loopback selection switch 17 c andturn off the first and second loopback selection switches 17 a, 17 b.

Thus, the control circuit 6 can selectively activate a particularloopback path by controlling the state of the switches.

The first transmit port TX1, the second transmit port TX2, and the thirdtransmit port TX3 can correspond to signals of various frequency rangesand/or signal types. Additionally, the amount of isolation provided byeach loopback path can be different. For example, the loopback impedanceof a loopback path (for example, loopback resistors 9 a-9 c) can beimplemented to achieve a desired amount of isolation. Accordingly,multiple loopbacks of different isolation, different operatingfrequency, and/or different signaling types can share a common backswitch.

In one embodiment, the first transmit port TX1 is a cellular transmitport, the second transmit port TX2 is a high power Bluetooth port, andthe third transmit port TX3 is a low power Bluetooth port.

Additional details of the front-end system 540 can be as describedearlier.

FIG. 10 is a schematic diagram of another embodiment of a front-endsystem 560 with multiple loopbacks and a shared back switch. Thefront-end system 560 includes an antenna port ANT, a first transmit portTX1, a second transmit port TX2, a receive port RX, a first transmitport DC blocking capacitor la, a second transmit port DC blockingcapacitor 1 b, a first antenna port DC blocking capacitor 2 a, a secondantenna port DC blocking capacitor 2 b, a first antenna switch 3 a, asecond antenna switch 3 b, a control circuit 6, a first loopbackresistor 9 a, a second loopback resistor 9 b, a receive port DC blockingcapacitor 13, a termination circuit 14, a first loopback DC blockingcapacitor 15 a, a second loopback DC blocking capacitor 15 b, a firstloopback selection switch 17 a, a second loopback selection switch 17 b,a shared back switch 18, an LNA input DC blocking capacitor 21, an LNAoutput DC blocking capacitor 22, an LNA input switch 23, an LNA outputswitch 24, and an LNA 25.

The front-end system 560 of FIG. 10 is similar to the front-end system500 of FIG. 8A, except that the front-end system 560 further includes areceive path from the antenna port ANT to the receive port RX via theLNA 25. The front-end systems herein can include a wide variety ofports, switches, paths, and circuitry. Although one specificimplementation of an LNA receive path is shown in FIG. 10, otherimplementations are possible.

Additional details of the front-end system 560 can be as describedearlier.

FIG. 11 is a schematic diagram of another embodiment of a front-endsystem 570 with multiple loopbacks and a shared back switch. Thefront-end system 570 includes an antenna port ANT, a first transmit portTX1, a second transmit port TX2, a receive port RX, a first transmitport DC blocking capacitor 1 a, a second transmit port DC blockingcapacitor 1 b, a first antenna port DC blocking capacitor 2 a, a secondantenna port DC blocking capacitor 2 b, a first antenna switch 33 a, asecond antenna switch 33 b, a control circuit 39, a first loopbackresistor 9 a, a second loopback resistor 9 b, a receive port DC blockingcapacitor 13, a termination circuit 44, a first loopback DC blockingcapacitor 15 a, a second loopback DC blocking capacitor 15 b, a firstloopback selection switch 571 a, a second loopback selection switch 571b, and a shared back switch 572.

The front-end system 570 of FIG. 11 is similar to front-end system 500of FIG. 8A, except that the front-end system 570 of FIG. 11 illustratesspecific termination circuit and switch implementations.

In particular, the termination circuit 44 includes a terminationresistor 71 and a termination capacitor 72 electrically connected inseries. Additionally, the illustrated first antenna switch 33 a includesa FET 51 a, a channel biasing resistor 52 a, a biasing inverter 53 a, aninverter output resistor 54 a, and a gate resistor 55 a. Furthermore,the illustrated second antenna switch 33 b includes a FET 51 b, achannel biasing resistor 52 b, a biasing inverter 53 b, an inverteroutput resistor 54 b, and a gate resistor 55 b. Additionally, theillustrated first loopback selection switch 571 a includes a FET 581 a,a channel biasing resistor 582 a, a biasing inverter 583 a, an inverteroutput resistor 584 a, and a gate resistor 585 a. Furthermore, theillustrated second loopback selection switch 571 b includes a FET 581 b,a channel biasing resistor 582 b, a biasing inverter 583 b, an inverteroutput resistor 584 b, and a gate resistor 585 b. Additionally, theshared back switch 572 includes a FET 591, a channel biasing resistor592, a biasing inverter 593, an inverter output resistor 594, and a gateresistor 595. Although specific switch implementations are shown, theteachings herein are applicable to switches implemented in a widevariety of ways.

In the illustrated embodiment, a first transmit selection control signalC_(TX1) from the control circuit 39 controls the first antenna switch 33a and the first loopback selection switch 571 a. Additionally, a secondtransmit selection control signal C_(Tx2) from the control circuit 39controls the second antenna switch 33 b and the second loopbackselection switch 571 b. Furthermore, a loopback control signal C_(LB)controls the shared back switch 572. In certain implementations, theloopback control signal C_(LB) is controlled based on a logical OR ofthe transmit selection control signals of the loopback paths. Forinstance, in an implementation with two loopback paths, the loopbackcontrol signal C_(LB) can controlled be controlled based on the Booleanequation C_(LB)=C_(TX1)+C_(TX2).

Additional details of the front-end system 570 can be as describedearlier.

Examples of Front-End Systems with Reactive Loopback Impedance and aShared Back Switch

A front-end system can include a combination of reactive loopbackimpedance and a shared back switch. Implementing a front-end system inthis manner can provide the benefits of low insertion loss in a transmitpath along with reduced area, complexity, and/or layout congestionassociated with a shared back switch.

Although one embodiment of a front-end system including both a reactiveloopback impedance and a shared back switch is illustrated, theteachings herein are applicable to a wide variety of configurations. Forexample, any of the front-end systems of FIGS. 1-5C can include multipleloopbacks using a shared back switch. Additionally, any of the front-endsystems of FIGS. 8A-11 can include reactive loopback impedance in one ormore of the loopback paths.

FIG. 12 is a schematic diagram of one embodiment of a front-end system600 with reactive loopback and with multiple loopbacks sharing a backswitch. The front-end system 600 includes an antenna port ANT, a firsttransmit port TX1, a second transmit port TX2, a receive port RX, afirst transmit port DC blocking capacitor 1 a, a second transmit port DCblocking capacitor 1 b, a first antenna port DC blocking capacitor 2 a,a second antenna port DC blocking capacitor 2 b, a first antenna switch3 a, a second antenna switch 3 b, a control circuit 6, a first reactiveloopback impedance 11 a, a second reactive loopback impedance 11 b, areceive port DC blocking capacitor 13, a termination circuit 14, a firstloopback DC blocking capacitor 15 a, a second loopback DC blockingcapacitor 15 b, a first loopback selection switch 17 a, a secondloopback selection switch 17 b, and a shared back switch 18.

Additional details of the front-end system 600 can be similar to thosedescribed earlier.

Examples Radio Frequency Systems Including a Front-End

FIG. 13A is a schematic diagram of one embodiment of a mobile device700. The mobile device 700 includes a front-end system 701, which caninclude a reactive loopback impedance and/or a shared back switch.Although not illustrated in FIG. 13A for clarity, the mobile device 700includes additional components and structures.

FIG. 13B is a schematic diagram of one embodiment of a base station 710.The base station 710 includes a front-end system 701, which can includea reactive loopback impedance and/or a shared back switch. Although notillustrated in FIG. 13B for clarity, the base station 710 includesadditional components and structures.

FIG. 14 is a schematic diagram of one embodiment of a radio frequencysystem 730. The RF system 730 includes a baseband processor 735, areceive circuit 742, a transmit circuit 746, a front-end system 701, andan antenna 759. The RF system 730 illustrates one example implementationof radio frequency circuitry suitable for operation in a mobile deviceor base station. However, mobile devices and base stations can beimplemented in a wide variety of ways.

The RF system 730 can be used for transmitting and/or receiving RFsignals using a variety of communication standards, including, forexample, Global System for Mobile Communications (GSM), Code DivisionMultiple Access (CDMA), wideband CDMA (W-CDMA), Long Term Evolution(LTE), Advanced LTE, 3G (including 3GPP), 4G, Enhanced Data Rates forGSM Evolution (EDGE), wireless local loop (WLL), and/or WorldwideInteroperability for Microwave Access (WiMax), as well as otherproprietary and non-proprietary communications standards.

The transmit circuit 746 and the receive circuit 742 can be used fortransmitting and receiving signals over the antenna 759. Although oneimplementation of the RF system 730 is illustrated in FIG. 14, the RFsystem 730 can be modified in any suitable manner. For example, the basestation 730 can be modified to include additional transmit circuits,receive circuits, front-ends, and/or antennas.

In the illustrated configuration, the receive circuit 742 includes adigital step attenuator (DSA) 732, a local oscillator 722, a first mixer723 a, a second mixer 723 b, a first programmable gain amplifier (PGA)725 a, a second PGA 725 b, a first filter 727 a, a second filter 727 b,a first analog-to-digital converter (ADC) 729 a, and a second ADC 729 b.Although one implementation of a receive circuit is illustrated in FIG.14, a receive circuit can include more or fewer components and/or adifferent arrangement of components.

An RF signal can be received on the antenna 759 and provided to thereceive circuit 742 using the front-end system 701. For example, thefront-end system 701 can be controlled to electrically couple theantenna 759 to an input of the DSA 732. In the illustrated embodiment,an amount of attenuation provided by the DSA 732 isdigitally-controllable, and can be set to achieve a desired signal powerlevel.

The first and second mixers 723 a, 723 b receive first and second localoscillator clock signals, respectively, from the local oscillator 722.The first and second local oscillator clock signals can have about thesame frequency and a phase difference equal to about a quarter of aperiod, or about 90°. The first and second mixers 723 a, 723 bdownconvert the output of the DSA 732 using the first and second localoscillator clock signals, respectively, thereby generating first andsecond demodulated signals. The first and second demodulated signals canhave a relative phase difference of about a quarter of a period, orabout 90°, and can correspond to an in-phase (I) receive signal and aquadrature-phase (Q) signal, respectively. In certain implementations,one of the first or second oscillator clock signals is generated byphase shifting from the other.

The first and second local oscillator clock signals can have a frequencyselected to achieve a desired intermediate frequency and/or basebandfrequency for the first and second demodulated signals. For example,multiplying the output of the DSA 732 by a sinusoidal signal from thelocal oscillator 722 can produce a mixed signal having a frequencycontent centered about the sum and difference frequencies of the carrierfrequency of the DSA output signal and the oscillation frequency of thelocal oscillator 722.

In the illustrated configuration, the first and second demodulatedsignals are amplified using the first and second programmable gainamplifiers 725 a, 725 b, respectively. To aid in reducing output noise,the outputs of the first and second programmable gain amplifiers 725 a,725 b can be filtered using the first and second filters 727 a, 727 b,which can be any suitable filter, including, for example, low pass, bandpass, or high pass filters. The outputs of the first and second filters727 a, 727 b can be provided to the first and second ADCs 729 a, 729 b,respectively. The first and second ADCs 729 a, 729 b can have anysuitable resolution. In the illustrated configuration, the outputs ofthe first and second ADCs 729 a, 729 b are provided to the basebandprocessor 735 for processing.

The baseband processor 735 can be implemented in a variety of ways. Forinstance, the baseband processor 735 can include a digital signalprocessor, a microprocessor, a programmable core, the like, or anycombination thereof. Moreover, in some implementations, two or morebaseband processors can be included in the RF system 730.

As shown in FIG. 14, the transmit circuit 746 receives data from thebaseband processor 735 and is used to transmit RF signals via theantenna 759. The transmit circuit 746 and the receive circuit 742 bothoperate using the antenna 759, and access to the antenna 759 iscontrolled using the front-end system 701. The illustrated transmitcircuit 746 includes first and second digital-to-analog converters(DACs) 737 a, 737 b, first and second filters 739 a, 739 b, first andsecond mixers 741 a, 741 b, a local oscillator 743, a combiner 745, aDSA 732, an output filter 751, and a power amplifier 758. Although oneimplementation of a transmit circuit is illustrated in FIG. 14, atransmit circuit can include more or fewer components and/or a differentarrangement of components.

The baseband processor 735 can output a digital in-phase (I) signal anda digital quadrature-phase (Q) signal, which can be separately processeduntil they are combined using the combiner 745. The first DAC 737 aconverts the digital I signal into an analog I signal, and the secondDAC 737 b converts the digital Q signal into an analog Q signal. Thefirst and second DACs 737 a, 737 b can have any suitable precision. Theanalog I signal and the analog Q signal can be filtered using the firstand second filters 739 a, 739 b, respectively. The outputs of the firstand second filters 739 a, 739 b can be upconverted using the first andsecond mixers 741 a, 741 b, respectively. For example, the first mixer741 a is used to upconvert the output of the first filter 739 a based onan oscillation frequency of the local oscillator 743, and the secondmixer 741 b is used to upconvert the output of the second filter 739 bbased on the oscillation frequency of the local oscillator 743.

The combiner 745 combines the outputs of the first and second mixers 741a, 741 b to generate a combined RF signal. The combined RF signal isprovided to an input of the DSA 732, which is used to control a signalpower level of the combined RF signal.

The output of the DSA 732 can be filtered using the output filter 751,which can be, for example, a low pass, band pass, or high pass filterconfigured to remove noise and/or unwanted frequency components from thesignal. The output of the output filter 751 can be amplified by a poweramplifier 758. In some implementations, the power amplifier 758 includesa plurality of stages cascaded to achieve a target gain. The poweramplifier 758 can provide an amplified RF signal to the antenna 759through the front-end system 701.

The front-end system 701 of FIG. 14 can be implemented using one or morefeatures discloses herein. Although FIG. 14 illustrates one example ofan RF system that can include a front-end system implemented inaccordance with the teachings herein, the front-end systems herein canbe used in other configurations of electronics.

FIG. 15A is a schematic diagram of one embodiment of a packaged module800. FIG. 15B is a schematic diagram of a cross-section of the packagedmodule 800 of FIG. 15A taken along the lines 15B-15B.

The packaged module 800 includes a semiconductor die 802, surface mountcomponents 803, wirebonds 808, a package substrate 820, andencapsulation structure 840. The package substrate 820 includes pads 806formed from conductors disposed therein. Additionally, the semiconductordie 802 includes pins or pads 804, and the wirebonds 808 have been usedto connect the pads 804 of the die 802 to the pads 806 of the packagesubstrate 820.

The semiconductor die 802 implements a front-end system that includes atleast one of a reactive loopback impedance 11 or a shared back switch18.

The package substrate 820 can be configured to receive a plurality ofcomponents such as the semiconductor die 802 and the surface mountcomponents 803, which can include, for example, surface mount capacitorsand/or inductors. In certain implementations, the packaged module 800includes multiple semiconductor dies attached to the package substrate820.

As shown in FIG. 15B, the packaged module 800 is shown to include aplurality of contact pads 832 disposed on the side of the packagedmodule 800 opposite the side used to mount the semiconductor die 802.Configuring the packaged module 800 in this manner can aid in connectingthe packaged module 800 to a circuit board, such as a phone board of awireless device. The example contact pads 832 can be configured toprovide radio frequency signals, bias signals, and/or power (forexample, a power supply voltage and ground) to the semiconductor die 802and/or the surface mount components 803. As shown in FIG. 15B, theelectrical connections between the contact pads 832 and thesemiconductor die 802 can be facilitated by connections 833 through thepackage substrate 820. The connections 833 can represent electricalpaths formed through the package substrate 820, such as connectionsassociated with vias and conductors of a multilayer laminated packagesubstrate.

In some embodiments, the packaged module 800 can also include one ormore packaging structures to, for example, provide protection and/orfacilitate handling. Such a packaging structure can include overmold orencapsulation structure 840 formed over the package substrate 820 andthe components and die(s) disposed thereon.

It will be understood that although the packaged module 800 is describedin the context of electrical connections based on wirebonds, one or morefeatures of the present disclosure can also be implemented in otherpackaging configurations, including, for example, flip-chipconfigurations.

FIG. 16 is a schematic diagram of another embodiment of a mobile device900. The mobile device 900 includes a baseband system 901, a transceiver902, a front-end system 903, an antenna 904, a power management system905, a memory 906, a user interface 907, and a battery 908.

The mobile device 900 can be used communicate using a wide variety ofcommunications technologies, including, but not limited to, 2G, 3G, 4G(including LTE, LTE-Advanced, and LTE-Advanced Pro), 5G NR, WLAN (forinstance, Wi-Fi), WPAN (for instance, Bluetooth and ZigBee), WMAN (forinstance, WiMax), and/or GPS technologies.

The transceiver 902 generates RF signals for transmission and processesincoming RF signals received from the antenna 904. It will be understoodthat various functionalities associated with the transmission andreceiving of RF signals can be achieved by one or more components thatare collectively represented in FIG. 16 as the transceiver 902. In oneexample, separate components (for instance, separate circuits or dies)can be provided for handling certain types of RF signals.

The front-end system 903 aids is conditioning signals transmitted toand/or received from the antenna 904. In the illustrated embodiment, thefront-end system 903 includes one or more of a power amplifier (PA) 911,a low noise amplifier (LNA) 912, a filter 913, switches 914, a reactiveloopback impedance 11 and/or a shared back switch 18. However, otherimplementations are possible.

For example, the front-end system 903 can provide a number offunctionalities, including, but not limited to, amplifying signals fortransmission, amplifying received signals, filtering signals, switchingbetween different bands, switching between different power modes,switching between transmission and receiving modes, duplexing ofsignals, multiplexing of signals (for instance, diplexing ortriplexing), or some combination thereof.

In certain implementations, the mobile device 900 supports carrieraggregation, thereby providing flexibility to increase peak data rates.Carrier aggregation can be used for both Frequency Division Duplexing(FDD) and Time Division Duplexing (TDD), and may be used to aggregate aplurality of carriers or channels. Carrier aggregation includescontiguous aggregation, in which contiguous carriers within the sameoperating frequency band are aggregated. Carrier aggregation can also benon-contiguous, and can include carriers separated in frequency within acommon band or in different bands.

The antenna 904 can correspond to one or more antennas used for a widevariety of types of communications. For example, the mobile device 900can include antennas for transmitting and/or receiving signalsassociated with a wide variety of frequencies and communicationsstandards.

In certain implementations, the mobile device 900 includes antennas thatsupport MIMO communications and/or switched diversity communications.For example, MIMO communications use multiple antennas for communicatingmultiple data streams over a single radio frequency channel. MIMOcommunications benefit from higher signal to noise ratio, improvedcoding, and/or reduced signal interference due to spatial multiplexingdifferences of the radio environment. Switched diversity refers tocommunications in which a particular antenna is selected for operationat a particular time. For example, a switch can be used to select aparticular antenna from a group of antennas based on a variety offactors, such as an observed bit error rate and/or a signal strengthindicator.

The mobile device 900 can operate with beamforming in certainimplementations. For example, the front-end system 903 can include phaseshifters having variable phase controlled by the transceiver 902.Additionally, the phase shifters are controlled to provide beamformation and directivity for transmission and/or reception of signals.

The baseband system 901 is coupled to the user interface 907 tofacilitate processing of various user input and output (I/O), such asvoice and data. The baseband system 901 provides the transceiver 902with digital representations of transmit signals, which the transceiver902 processes to generate RF signals for transmission. The basebandsystem 901 also processes digital representations of received signalsprovided by the transceiver 902. As shown in FIG. 16, the basebandsystem 901 is coupled to the memory 906 of facilitate operation of themobile device 900.

The memory 906 can be used for a wide variety of purposes, such asstoring data and/or instructions to facilitate the operation of themobile device 900 and/or to provide storage of user information.

The power management system 905 provides a number of power managementfunctions of the mobile device 900. In certain implementations, thepower management system 905 includes a PA supply control circuit thatcontrols the supply voltages of the power amplifiers 911. For example,the power management system 905 can be configured to change the supplyvoltage(s) provided to the power amplifier 911 to improve efficiency,such as power added efficiency (PAE).

As shown in FIG. 16, the power management system 905 receives a batteryvoltage from the battery 908. The battery 908 can be any suitablebattery for use in the mobile device 900, including, for example, alithium-ion battery.

Front-end systems can be included in various electronic devices,including, but not limited to consumer electronic products, parts of theconsumer electronic products, electronic test equipment, etc. Examplesof the electronic devices can also include, but are not limited to,circuits of communication networks. The consumer electronic products caninclude, but are not limited to, a mobile phone, a tablet, a television,a computer monitor, a computer, a hand-held computer, a personal digitalassistant (PDA), a microwave, a refrigerator, an automobile, a stereosystem, a cassette recorder or player, a DVD player, a CD player, a VCR,an MP3 player, a radio, a camcorder, a camera, a digital camera, aportable memory chip, a washer, a dryer, a washer/dryer, a copier, afacsimile machine, a scanner, a multi-functional peripheral device, awrist watch, a clock, etc. Further, the electronic devices can includeunfinished products.

CONCLUSION

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense, as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” The word “coupled”, as generally usedherein, refers to two or more elements that may be either directlyconnected, or connected by way of one or more intermediate elements.Likewise, the word “connected”, as generally used herein, refers to twoor more elements that may be either directly connected, or connected byway of one or more intermediate elements. Additionally, the words“herein,” “above,” “below,” and words of similar import, when used inthis application, shall refer to this application as a whole and not toany particular portions of this application. Where the context permits,words in the above Detailed Description using the singular or pluralnumber may also include the plural or singular number respectively. Theword “or” in reference to a list of two or more items, that word coversall of the following interpretations of the word: any of the items inthe list, all of the items in the list, and any combination of the itemsin the list.

Moreover, conditional language used herein, such as, among others,“can,” “could,” “might,” “can,” “e.g.,” “for example,” “such as” and thelike, unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements and/or states. Thus, such conditional language is notgenerally intended to imply that features, elements and/or states are inany way required for one or more embodiments or that one or moreembodiments necessarily include logic for deciding, with or withoutauthor input or prompting, whether these features, elements and/orstates are included or are to be performed in any particular embodiment.

The above detailed description of embodiments of the invention is notintended to be exhaustive or to limit the invention to the precise formdisclosed above. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. For example, whileprocesses or blocks are presented in a given order, alternativeembodiments may perform routines having steps, or employ systems havingblocks, in a different order, and some processes or blocks may bedeleted, moved, added, subdivided, combined, and/or modified. Each ofthese processes or blocks may be implemented in a variety of differentways. Also, while processes or blocks are at times shown as beingperformed in series, these processes or blocks may instead be performedin parallel, or may be performed at different times.

The teachings of the invention provided herein can be applied to othersystems, not necessarily the system described above. The elements andacts of the various embodiments described above can be combined toprovide further embodiments.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the disclosure. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the disclosure. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the disclosure.

What is claimed is:
 1. A front-end system comprising: an antenna port; atransmit port configured to receive a transmit signal; an antenna switchelectrically connected along a transmit path between the transmit portand the antenna port; a receive port; and a loopback circuit including areactive loopback impedance and a back switch electrically connected inseries in a loopback path between the transmit port and the receiveport, the loopback circuit configured to provide a portion of thetransmit signal to the receive port when the back switch is activated,the loopback circuit further including a receive port DC blockingcapacitor, the back switch electrically connected between the reactiveloopback impedance and the receive port DC blocking capacitor.
 2. Thefront-end system of claim 1 wherein the reactive loopback impedanceincludes a capacitor.
 3. The front-end system of claim 1 wherein thereactive loopback impedance includes two or more capacitors electricallyconnected in series.
 4. The front-end system of claim 1 wherein theloopback path does not include any explicit resistors.
 5. The front-endsystem of claim 1 wherein the loopback circuit has an impedance angle ofat least 10 degrees at a frequency of interest.
 6. The front-end systemof claim 5 wherein the frequency of interest is a carrier frequency ofthe transmit signal.
 7. The front-end system of claim 1 furthercomprising a control circuit configured to control the antenna switchand the back switch.
 8. The front-end system of claim 1 furthercomprising a low noise amplifier electrically connected between theantenna port and the receive port in a receive path.
 9. The front-endsystem of claim 1 wherein the loopback circuit includes a terminationcircuit electrically connected between the back switch and ground. 10.The front-end system of claim 1 further comprising an antenna DCblocking capacitor and a transmit port DC blocking capacitor, theantenna DC blocking capacitor, the antenna switch, and the transmit portDC blocking capacitor electrically connected in series between theantenna port and the transmit port.
 11. A method of providing transmitloopback in a front-end system, the method comprising: receiving atransmit signal at a transmit port; providing the transmit signal to anantenna port via a transmit path that includes an antenna switch;controlling an amount of isolation between the transmit path and areceive port using a loopback circuit, the loopback circuit including areactive loopback impedance and a back switch electrically connected inseries between the transmit port and the receive port; providing aportion of the transmit signal to the receive port when the back switchis activated; and providing DC blocking using a receive port DC blockingcapacitor, the back switch electrically connected between the reactiveloopback impedance and the receive port DC blocking capacitor.
 12. Themethod of claim 11 wherein controlling the amount of isolation betweenthe transmit path and the receive port includes using the loopbackcircuit to provide an impedance angle of at least 10 degrees at acarrier frequency of the transmit signal.
 13. A wireless communicationdevice comprising: an antenna; a receive circuit configured to process areceive signal; a transmit circuit configured to generate a transmitsignal; and a front-end system including an antenna port electricallyconnected to the antenna, a transmit port configured to receive thetransmit signal from the transmit circuit, a receive port configured toprovide the receive signal to the receive circuit, and a loopbackcircuit including a reactive loopback impedance and a back switchelectrically connected in series between the transmit port and thereceive port, the loopback circuit configured to provide a portion ofthe transmit signal to the receive port when the back switch isactivated, the front-end system further including an antenna DC blockingcapacitor, an antenna switch, and a transmit port DC blocking capacitorelectrically connected in series between the antenna port and thetransmit port.
 14. The wireless communication device of claim 13 whereinthe reactive loopback impedance includes a capacitor.
 15. The wirelesscommunication device of claim 13 wherein the reactive loopback impedanceincludes two or more capacitors electrically connected in series. 16.The wireless communication device of claim 13 wherein the loopback pathdoes not include any explicit resistors.
 17. The wireless communicationdevice of claim 13 wherein the loopback circuit has an impedance angleof at least 10 degrees at a carrier frequency of the transmit signal.18. The wireless communication device of claim 13 wherein the front-endsystem further includes a low noise amplifier electrically connectedbetween the antenna port and the receive port in a receive path.
 19. Themethod of claim 11 further comprising providing DC blocking using anantenna DC blocking capacitor, the antenna switch electrically connectedbetween the antenna DC blocking capacitor and the reactive loopbackimpedance.
 20. The wireless communication device of claim 13 wherein theloopback circuit includes a termination circuit electrically connectedbetween the back switch and ground.